The present invention relates in general to ohmic contacts useful in semiconductor devices, and more particularly, to a hypoeutectic ohmic contact which is improved by the addition of a diffusion barrier.
Low resistivity reliable ohmic contacts to n-GaAs have been previously implemented using a eutectic combination of gold and germanium. In a previously filed co-pending application, Serial No. 769,031, there has been described a hypoeutectic ohmic contact to gallium arsenide. The contact which results from this hypoeutectic ohmic contact process has a gold surface which retains its smooth surface morphology and is consistently reproducible by an easily controllable process. While the resultant contact has the required low resistivity and physical characteristics to provide a good electrical contact to a GaAs surface, there are other characteristics which make this type of contact undesirable for use with an E-beam lithography process.
In certain manufacturing processes the metal layer which is deposited as an ohmic contact on a GaAs surface is used not only to provide a good electrical contact to the layer but in addition is used as a key to provide alignment during certain E-beam lithography processes. In using the ohmic contact as a key during an alignment step, the E-beam signal senses the difference in reflected energy from the gold surface of the ohmic contact and the reflected energy from the GaAs surface. Any gallium appearing in the gold portion of the ohmic contact would therefore degrade the resolution of the E-beam signal when differentiating between the ohmic contact gold and the GaAs surface. One of the characteristics of the hypoeutectic process above described is that gallium outdiffuses from the gallium arsenide surface into the gold layer. In some cases this outdiffusion is to such an extent that the gallium is clearly visible during E-beam lithography and therefore degrades the resolution of the E-beam lithography process.
A previously known process to alleviate this problem is to add a second layer comprising titanium platinum and gold to the ohmic contact layer. When this second layer is added the platinum portion acts as a barrier and prevents the gallium from the GaAs surface from penetrating through to the outer surface of the gold layer. However, this involves a second manufacturing step which is conducted at a different temperature under different conditions from the formation of the hypoeutectic contact. In addition, this second step would normally take place after the manufacturing step during which the E-beam lithography would be used and therefore would not be feasible for reducing the amount of gallium visible to the E-beam signal in the gold portion of the hypoeutectic ohmic contact layer. It would therefore be desirable to have an improved hypoeutectic contact with reduced gallium outdiffusion from the underlying gallium arsenide layer. It would also be desirable if this improved hypoeutectic contact could be formed in a one step process which is accomplished prior to a E-beam lithography.
Accordingly, is an object of the present invention to provide an improved hypoeutectic ohmic contact for use on semiconductor surfaces.
Another object of the present invention is to provide an improved hypoeutectic ohmic contact having a barrier layer.
Yet another object of the present invention is to provide a hypoeutectic ohmic contact with a barrier layer which can be produced in a single manufacturing process step.
A still further object of the present invention is to provide a hypoeutectic ohmic contact by a process which is suitable for use prior to E-beam lithography.